1. Field of the Invention
This invention relates to the liquefaction of coal and similar carbonaceous solids and is particularly concerned with hydrogen-donor solvent liquefaction operations.
2. Description of the Prior Art
Hydrogen-donor solvent processes are among the most promising methods for the production of liquid hydrocarbons from coal and similar carbonaceous solids. In processes of this type, the coal or other feed material is contacted with molecular hydrogen and a hydrogen-donor solvent at elevated temperature and pressure in a liquefaction zone. High molecular weight constituents of the coal are broken down or cracked and hydrogenated to form lower molecular weight gaseous, vaporous and liquid products. The hydrogen-donor solvent and molecular hydrogen contributes hydrogen atoms which react with free radicals liberated from the coal and prevent their recombination. The molecular hydrogen also serves as replacement hydrogen for depleted hydrogen-donor molecules in the solvent and results in the formation of additional hydrogen-donor molecules by in situ hydrogenation. The effluent from the liquefaction zone is processed for the recovery of gases and vapors, liquid hydrocarbon in the solvent boiling range, and heavier constituents including a bottoms fraction containing suspended particles of unreacted coal, mineral matter and other solid residues. The solvent boiling range materials are further treated in a catalytic solvent hydrogenation zone to generate hydrogen-donor solvent for recycle to the liquefaction zone and produce additional liquid products.
The solvent boiling range material sent to the solvent hydrogenation unit in processes of the type described above are highly aromatic. The partial hydrogenation of these materials over a catalyst such as cobalt molybdate produces hydrogenated aromatics such as indane, C.sub.10 -C.sub.12 tetrahydronaphthalenes, C.sub.12 and C.sub.13 acenaphthenes, di-, tetra- and octahydroanthracenes, tetrahydroacenaphthenes, crysenes, phenanthrenes, pyrenes and the like. These hydrogenated aromatics contain donatable hydrogen which reacts with the free radicals as described above. Laboratory work and pilot plant studies have shown that the concentration of donatable hydrogen in the solvent must be maintained at a high level if high liquefaction liquid yields are to be obtained. If this concentration falls below about 2 weight percent, based on the feed coal to the liquefaction reactor, the liquefaction yields tend to be adversely affected.
It has been proposed that coal liquefaction operations be improved through the use of a hydrogenation catalyst in the liquefaction zone. The catalysts typically mentioned have been compounds of cobalt, molybdenum, nickel, tin, iron and similar metals deposited on carries such as alumina, magnesia, silica and the like. It has also been suggested that coal ash and similar materials which appear to have catalytic activity be used for this purpose. Various means for carrying out the liquefaction reaction in the presence of solid catalysts particles have been devised. It has also been suggested that the coal be impregnated with solutions of catalytically active material prior to the liquefaction step. In general, however, it has been found that catalysts are of only limited value in processes using hydrogen-donor solvents and that the added costs associated with the use of a catalyst may be difficult to justify.